I did the seat belt thing and my left knee and left side got beat up pretty badly. The biggest difference I noticed is that my upper body seemed more planted.

I did the seatbelt tightening thing for years in my R32, but the biggest issue I had was occasionally it would unlock itself (maybe the bmw's don't do that). Now that I have the harnesses, I can say it came nowhere close to holding me in as well, though it's the main non-harness alternative. It is definitely not a cheap upgrade, if you go the full cage (or half cage like me), seat, and harness route.